Anatoly reports that since commit:

  ff5b4f1e ("locking/atomic: sparc: move to ARCH_ATOMIC")

... it's possible to reliably trigger an oops by running:

  stress-ng -v --mmap 1 -t 30s

... which results in a NULL pointer dereference in
__split_huge_pmd_locked().

The underlying problem is that commit ff5b4f1e left
arch_cmpxchg64_local() defined in terms of cmpxchg_local() rather than
arch_cmpxchg_local(). In <asm-generic/atomic-instrumented.h> we wrap
these with macros which use identically-named variables. When
cmpxchg_local() nests inside cmpxchg64_local(), this casues it to use an
unitialized variable as the pointer, which can be NULL.

This can also be seen in pmdp_establish(), where the compiler can
generate the pointer with a `clr` instruction:

0000000000000360 <pmdp_establish>:
 360:   9d e3 bf 50     save  %sp, -176, %sp
 364:   fa 5e 80 00     ldx  [ %i2 ], %i5
 368:   82 10 00 1b     mov  %i3, %g1
 36c:   84 10 20 00     clr  %g2
 370:   c3 f0 90 1d     casx  [ %g2 ], %i5, %g1
 374:   80 a7 40 01     cmp  %i5, %g1
 378:   32 6f ff fc     bne,a   %xcc, 368 <pmdp_establish+0x8>
 37c:   fa 5e 80 00     ldx  [ %i2 ], %i5
 380:   d0 5e 20 40     ldx  [ %i0 + 0x40 ], %o0
 384:   96 10 00 1b     mov  %i3, %o3
 388:   94 10 00 1d     mov  %i5, %o2
 38c:   92 10 00 19     mov  %i1, %o1
 390:   7f ff ff 84     call  1a0 <__set_pmd_acct>
 394:   b0 10 00 1d     mov  %i5, %i0
 398:   81 cf e0 08     return  %i7 + 8
 39c:   01 00 00 00     nop

This patch fixes the problem by defining arch_cmpxchg64_local() in terms
of arch_cmpxchg_local(), avoiding potential shadowing, and resulting in
working cmpxchg64_local() and variants, e.g.

0000000000000360 <pmdp_establish>:
 360:   9d e3 bf 50     save  %sp, -176, %sp
 364:   fa 5e 80 00     ldx  [ %i2 ], %i5
 368:   82 10 00 1b     mov  %i3, %g1
 36c:   c3 f6 90 1d     casx  [ %i2 ], %i5, %g1
 370:   80 a7 40 01     cmp  %i5, %g1
 374:   32 6f ff fd     bne,a   %xcc, 368 <pmdp_establish+0x8>
 378:   fa 5e 80 00     ldx  [ %i2 ], %i5
 37c:   d0 5e 20 40     ldx  [ %i0 + 0x40 ], %o0
 380:   96 10 00 1b     mov  %i3, %o3
 384:   94 10 00 1d     mov  %i5, %o2
 388:   92 10 00 19     mov  %i1, %o1
 38c:   7f ff ff 85     call  1a0 <__set_pmd_acct>
 390:   b0 10 00 1d     mov  %i5, %i0
 394:   81 cf e0 08     return  %i7 + 8
 398:   01 00 00 00     nop
 39c:   01 00 00 00     nop

Fixes: ff5b4f1e ("locking/atomic: sparc: move to ARCH_ATOMIC")
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Anatoly Pugachev <matorola@gmail.com>
Link: https://lore.kernel.org/r/20210707083032.567-1-mark.rutland@arm.com